Capability for users to create batch difference plots

[christinaholtNOAA]

Users now have the option to create a "diff" plot using create_graphics.py. It will accept two directories with two options for file name templates, and will create all the difference maps for variables specified in the image list. All other behaviors are similar to regular map plotting (forecast hours, regions, etc.).

There are a few things it won't do out of the box:

• Plot differences from different models. HRRR and RRFS, for example.
• Plot graphics accumulated variables. It will plot the variables labeled as accumulated in UPP, though.
• Have a uniform color scale across variables. Each plot will assess the min/max values of the data field and use those numbers to set the value range.
• Use different color maps. That's hard coded.

Here is an example plot:

I'll add some additional comments to the code after the PR is opened.

[christinaholtNOAA]

@Brian-Jamison I am not totally convinced all variables will work given the difference in naming convention between the two models -- HRRR and RRFS. It might be true that for some select variables that do share the same variable names, these might work, but I don't want to claim that it certainly would.